Refactor TypeContext locals to HashMap

Change the `slots` field in `TypeContext` from a `Vec` to a `HashMap`.
This allows for sparse local variable storage and avoids the need to
pre-allocate for all slots, which is more efficient when not all locals
are used.

src/ast/compiler/type_checker.rs

@@ -1,13 +1,14 @@
 use crate::ast::compiler::bound_nodes::{Address, BoundKind, Node, TypedNode};
 use crate::ast::diagnostics::Diagnostics;
 use crate::ast::types::StaticType;
+use std::collections::HashMap;
 use std::rc::Rc;
 
 /// Manages the types of locals and upvalues during a single type-checking pass.
 struct TypeContext<'a> {
     _parent: Option<&'a TypeContext<'a>>,
     /// Maps slot index -> Inferred Type
-    slots: Vec<StaticType>,
+    slots: HashMap<u32, StaticType>,
     /// Types of captured variables (passed from outer scope)
     upvalue_types: Vec<StaticType>,
     /// Access to root types for unified resolution
@@ -18,14 +19,14 @@ struct TypeContext<'a> {
 
 impl<'a> TypeContext<'a> {
     fn new(
-        slot_count: u32,
+        _slot_count: u32,
         upvalue_types: Vec<StaticType>,
         root_types: &'a std::cell::RefCell<Vec<StaticType>>,
         parent: Option<&'a TypeContext<'a>>,
     ) -> Self {
         Self {
             _parent: parent,
-            slots: vec![StaticType::Any; slot_count as usize],
+            slots: HashMap::new(),
             upvalue_types,
             root_types,
             current_params_ty: None,
@@ -36,7 +37,7 @@ impl<'a> TypeContext<'a> {
         match addr {
             Address::Local(slot) => self
                 .slots
-                .get(slot.0 as usize)
+                .get(&slot.0)
                 .cloned()
                 .unwrap_or(StaticType::Any),
             Address::Global(idx) => self
@@ -56,9 +57,7 @@ impl<'a> TypeContext<'a> {
     fn set_type(&mut self, addr: Address, ty: StaticType) {
         match addr {
             Address::Local(slot) => {
-                if let Some(entry) = self.slots.get_mut(slot.0 as usize) {
-                    *entry = ty;
-                }
+                self.slots.insert(slot.0, ty);
             }
             Address::Global(idx) => {
                 let mut rt = self.root_types.borrow_mut();